A prior art optical fiber amplifier has a signal light source for generating a signal light and an exciting light source for generating an exciting light which are separately provided.
FIG. 3 is a block diagram showing an exemplary configuration of a prior art optical fiber amplifier. The prior art optical fiber amplifier comprises an inline isolator 3, a rare earth-doped optical fiber 4, an external modulator 5, a wave length combining coupler (multiplexer) 7, a signal light source 8 and an exciting light source 9.
In FIG. 3, the signal light source for generating the signal light is separated from the exciting light source for generating the exciting light and they are disposed in different positions. The signal light which is generated from the signal light source 8 is combined with the exciting light generated from the exciting light source 9 by the wave length combining coupler 7 and the combined signal light is passed through the inline isolator 3 and the rare earth-doped optical fiber 4 and is incident upon the external modulator 5.
The prior art optical fiber amplifying device disclosed in Japanese Patent Kokai Publication JP-A-9-160085 which is shown as an exemplary prior art comprises two discrete optical fiber amplifiers so that an optical fiber amplifying device of a two stage type or an independent bidirectional transmitting type is formed. In this prior art optical fiber amplifier, reduction in current flowing through the exciting light source, enhancement in gain and increase in output are achieved by increasing the exciting efficiency.
A prior art optical fiber amplifying device disclosed in Japanese Patent Kokai Publication JP-A-9-222621 which is shown as an exemplary prior art comprises a composite optical component having a capability of receiving the power of input signal light which is provided on the input side of the optical fiber amplifier. This optical fiber amplifying device is configured so that it has an exciting mechanism for transmitting the exciting light in a direction opposite to that of the signal light.
However, the following problems have been encountered in the course of investigations toward the present invention. Namely, since the signal light source is separated from the exciting light source in both the above-mentioned prior arts, a high cost is a problem, and the size entire optical fiber amplifier becomes larger since accommodating space in which the light sources are separately disposed is required.
It is therefore an object of the present invention to provide a novel optical fiber amplifier free of the aforementioned drawbacks. Particularly, it is a further object of the present invention to provide an optical fiber amplifier comprising signal light source and exciting light source which are integral with each other.
Other objects of the present invention will become apparent in the entire disclosure.
According to a first aspect of the present invention, there is provided an optical fiber amplifier having a signal light source transmitting a signal light and an exciting light source exciting the signal light, characterized in that the optical fiber amplifier comprises:
a light emitting element having a wide spectrum range,
a fiber grating having a diffraction grating adapted to reflect only signal light having a wave length which is on the longer wave length side than that (i.e. a center wave length) of the light emitting element, and the former wave length is in a range that can be amplified by the optical fiber amplifier,
an inline isolator having a high isolation ability in the same wave length range as that of said signal light, and
an optical fiber adapted to stimulatedly amplify the signal light.
The signal light source and the exciting light source are formed of the same light emitting element.
According to a second aspect of the present invention, in the optical fiber amplifier, the light emitting element is a Fabry-Perot semiconductor laser element.
According to a third aspect of the present invention, the optical fiber amplifier is characterized in that the Fabry-Perot semiconductor laser element has an oscillating wave length characteristic in the 1.48 .mu.m band and that the oscillated light has a wide spectrum of the 1.48 .mu.m band.
According to a fourth aspect of the present invention, the optical fiber amplifier is characterized in that the optical fiber is a rare earth doped optical fiber.
According to a fifth aspect of the present invention, the optical fiber amplifier is characterized in that the rare earth doped optical fiber is an optical fiber which is formed by adding any of rare earth elements into a host glass of quartz glass or fluoride glass.
According to a sixth aspect of the present invention, the optical fiber is characterized in that the optical fiber amplifier comprises a 1.55 .mu.m band signal light source module transmitting the signal light, and a 1.48 .mu.m band exciting light source exciting the rare earth doped optical fiber. The signal light source module and the exciting light source are formed as an integral module.
According to a seventh aspect of the present invention, the optical fiber amplifier is characterized in that the optical fiber amplifier further comprises an external modulator for modulating the signal light.
According to an eighth aspect of the present invention, the optical fiber amplifier is characterized in that the external modulator is disposed at a rear stage of the optical fiber, the external modulates modulating the signal light which has been amplified by the optical fiber.
According to a ninth aspect of the present invention, the optical fiber amplifier is characterized in that the external modulator is disposed between the fiber grating and the inline isolator. The optical fiber amplifier further includes a wave length-division demultiplexer and a wave length-division multiplexer which are disposed at upstream and downstream of said external modulator, respectively. The demultiplexer is optically connected with the multiplexer via a bypass circuit so that the separated lights which have been separated by the demultiplexer are combined with one another in the multiplexer via the bypass circuit.